1. Field of the Invention
The invention relates to a method for producing a thick, thermoplastically deformable fiber-reinforced semifinished product from a mixed non-woven in which thermoplastic fibers and reinforcing fibers are present.
2. Description of the Related Art
Thermoplastically deformable semifinished products which comprise reinforcing fibers, in particular glass fibers, are increasingly used for producing moldings, in particular for motor vehicle parts. These “plastic panels” have high toughness and strength. The semifinished GMT products are produced on a large industrial scale by uniting continuous glass mats and webs of thermoplastic melt on a twin-belt press. However, this mode of operation is energy-intensive, because the high-viscosity melt has to be pressed into the mat. Fiber contents of more than 50% by weight are rarely achievable by this method. Since the glass mats are generally composed of fiber bundles, the saturation process is never entirely complete and uniform, the result being the occurrence of regions of microscopic inhomogeneity, leading to high standard deviations of mechanical properties.
Another process used in industry is similar to paper-making. Thermoplastic fibers and reinforcing fibers are mixed with one another in the form of an aqueous slurry, the slurry is squeezed, and the resultant mixed non-woven is dried and hot-pressed. This involves large amounts of water, and the purification of waste water contaminated with auxiliaries.
DE-A 36 14 533 describes a process for producing moldings from thermoplastics which comprise an inserted reinforcement. This is similar to textile fiber technology in that a mixed non-woven composed of thermoplastic fibers and reinforcing fibers is produced by the carding or airlay process and, for example, consolidated by needling. Cut-to-size pieces from this mixed non-woven are heated and are directly, with no prior consolidation, pressed to give three-dimensional moldings. However, complete and thorough saturation is rarely achieved here, especially in the case of components of complicated shape, and the mechanical properties of the moldings are therefore unsatisfactory.
U.S. Pat. No. 4,948,661 describes the production of a consolidated semifinished product whose thickness is from 1.25 to 2.5 mm. First, a dry process produces a mixed non-woven from thermoplastic fibers and reinforcing fibers. However, this mixed non-woven is not needled, but is folded together in the manner of corrugations, and is consolidated directly through hot-pressing to give the semifinished product. Because there is no consolidation of the mixed non-woven, the only practical problem-free consolidation method is batchwise consolidation. Although mention is also made of continuous consolidation on a twin-belt press, this would be associated with the disadvantages described above—if indeed it is feasible in practice.
Finally, EP-A 555 345 describes an air-permeable fiber structure made from a mixed non-woven produced by a wet or dry method and composed of thermoplastic fibers and reinforcing fibers. This mixed non-woven, which has not been needled, is partially consolidated by careful incipient melting of the thermoplastic fibers, by bonding these to the reinforcing fibers at the intersections. Continuous production of the fiber structure is not described. Another disadvantage of the process is that the non-wetted reinforcing fibers can corrode during storage, and here again it is difficult to obtain complete and thorough saturation during the production of moldings.
The German patent application P 101 05 813.6 describes a process for producing a thermoplastically deformable, fiber-reinforced semifinished product with a thickness of from 0.2 to 3.0 mm, by needling a mixed non-woven composed of thermoplastic fibers and of reinforcing fibers of length from 30 to 300 mm, and heating the mixed non-woven, and pressing it on a calender or in a polishing stack. If the intention is to press this relatively thin semifinished product to produce finished parts with a complicated three-dimensional shape, in particular those having filigree ribs, the ribs are found not to have a satisfactory filling of fibers. In the case of a thicker semifinished product, improved flowability should be assumed.